Optical communication typically involves the transmission of light e.g. laser light in the wavelength range 800 nm to 1600 nm, over an optical pathway generally including optical fiber and suitable active and passive processing devices such as filters, amplifiers, cross connects, add-drop modules, and dispersion compensation units. A variety of protocols are employed to standardize the form of an optical signal. In a typical protocol, the signal is broken into time slots of between 1000 and 25 picoseconds. A digital 0 is represented by the substantial absence of detectable light during the time slot while a digital 1 corresponds to the presence of such light. Thus, as shown in FIG. 1 an exemplary signal might have the form shown with the corresponding digital signal above the light signal. Although the signal upon insertion into the optical system generally has a square wave pattern as demonstrated in FIG. 1 at 12, this pattern is corrupted both by traversal of the optical fiber and by interaction with the various active and passive devices in the optical system. Thus after such traversal the signal might have degraded as shown by the dotted waveform 10 in FIG. 1. Clearly, if this process continues the signal to noise ratio of the signal degrades and it will not be extractable from the surrounding noise. Thus, it is desirable to periodically reconstitute the wave shape and to amplify this reconstituted wave shape so that the original signal form as inserted in the system is replicated. The process of reshaping the waveform is generally denominated equalization.
In the equalization process not only is the shape of the waveform a concern, but also the nature of transmission on the carrier is also a significant concern. For example, as shown in FIG. 2 the waveform 36 in FIG. 1 is magnified in FIG. 2 to show the underlying carrier light 35. Thus the sine wave associated with electromagnetic carrier frequency of the signal has a specific perceived wavelength as shown in FIG. 2 across the profile of each pulse of light forming the optical signal. The perceived wavelength is the result of the summation of the frequency components that make up the pulse. As injected the perceived wavelength/carrier of this underlying carrier is typically constant throughout the pulse. However, the various distortions imposed on the signal also frequently cause this perceived carrier frequency to become non-constant. Thus as shown in FIG. 3, the distorted signal 36 for some forms of distortion has an underlying carrier with a varying perceived frequency e.g. the perceived frequency of the carrier 25 decreases (and thus the wavelength increases) starting in region 37 and continuing through region 38. This variation in the frequency of the underlying character is generally termed chirp. For many applications, e.g. ultra long haul transmission, signal chirp should be controlled for improved performance.
Thus to equalize the signal it is desirable to return it to its original shape, e.g. a square wave, and it is often desirable to either carefully control, or remove any chirp present. If no chirp is present avoiding the introduction of chirp during the equalization process is often desirable. These goals for equalization are complicated by the numerous systems presently employed for optical communication. For example, many systems use different time slot lengths. That is, the repetition rate (i.e. the bit rate) is generally lower in older systems relative to newer systems. For example, many systems that have been deployed for many years have a 2.5 Gb/s bit rate compared to many current systems having a 10 Gb/s or 40 Gb/s repetition rate. Additionally, many optical fiber pathways are subjected to signals of different repetition rates. It is therefore advantageous for a device designed to equalize a signal having one repetition rate not to produce an unacceptable affect on a signal with a different repetition rate. It is further desirable for the device to be tunable to allow flexibility in application.
The most common approach for equalization involves a two-stage device such as shown in FIG. 4. In this device, an incoming distorted wave is injected into waveguide 42. A small fraction (generally between 0.1 and 15 power percent) is diverted into waveguide 43 and delayed relative to the signal on wavepath 42 until it is then again injected at 44. In one circumstance, if the delay time is chosen appropriately, the two waveforms (one 47 on wavepath 43 and one 49 on wavepath 42) interact constructively as shown in FIG. 5. The combined pulse shown by dotted line 56 continues on wavepath 48 and a second small portion (typically 0.1 to 25 power percent) is diverted into wavepath 42. This diverted portion is advanced relative to that in wavepath 48 so that the two pulses combined at 46 interact constructively as shown in FIG. 6 to give the resultant curve shown by dotted line 50.
In this manner, the waveform is equalized acceptably. However, the wavepaths 48 and 43 used to provide the necessary time delays are relatively large constructs typically including between 1 and 20 centimeters of a monolithic optical waveguide as the delay path length. Thus such devices are relatively large. Additionally, the devices are constructed to process signals of a single repetition rate. That is, the time delays are established for a specific repetition rate to provide constructive (or destructive) interference at 44 and at 46 in FIG. 4. A signal having a different repetition rate requires a different delay time to provide appropriate interference. Thus such equalization devices unacceptably distort signals having a repetition rate significantly different from those intended to be processed. Finally, as discussed, such devices are fabricated for a specific repetition rate and are not adjustable for other repetition rates.
Thus it would be desirable to have an equalization device that is relatively compact, does not adversely affect signals deviating from those for which the device is configured, and which is tunable to equalize signals having different characteristics such as different repetition rates.